Portable electronics products such as cellular phone, PDA, DVC, DSC acquire multi-functions in an accelerating speed, and in such condition the miniaturization and the weight reduction is essential for these products to be accepted by the market, and thus LSI (large scale integration) having higher integration level are necessary to satisfy these needs.
On the other hand, more user-friendly and convenient products are also desired for these electronics products by the market, and thus LSI having multiple functions and higher performances are needed for utilizing these products. For this reason, number of inputs/outputs (I/O) increases as the level of the integration of LSI increases, while the requirement for miniaturizing the package itself becomes considerable.
In order to combine these two requirements into one product, developments of the semiconductor packaging, which is applicable for mounting the semiconductor devices at higher density of devices, is strongly demanded. To address these requirements, various developments in the packaging technology called “chip size packaging” or “chip scale packaging” (CSP) are actively conducted.
An integrated circuit die or a chip, is a small device formed on a silicon wafer, such as a semiconductor wafer. The integrated circuit die usually is made from a semiconductor material such as silicon or gallium arsenide. Such an integrated circuit die is typically cut from the wafer and attached to a substrate or base carrier for redistribution of interconnects.
Integrated circuit dies have input/output pads that must be connected to external circuitry in order to function as part of an electronic system. The connection media is typically an array of metallic leads (e.g., a lead frame) or a support circuit (e.g., a substrate), although the connection can be made directly to a circuit panel (e.g., a mother board).
Flip-chip bonding involves providing pre-formed solder bumps on the pads, flipping the chip or integrated circuit so that the pads face down and are aligned with and contact matching bond sites, and melting the solder bumps to wet the pads and the bond sites.
The attachment of a flip chip to a substrate such as printed circuit board involves aligning the solder bumps on the flip chip with a plurality of contact points (configured to be a mirror image of the solder ball arrangement on the flip chip) on a facing surface of the substrate. A plurality of solder bumps may also be formed on a facing surface of the substrate at the contact points.
To form the solder bumps reliably, wettable pads on the substrate are defined. The substrate pads are metallized to be wettable by the solder. Plated-copper metallization offers the lowest cost with great electrical conductivity. During the formation of the solder bumps, the solder wets only the wettable pads on the substrate. If wettable pads are not defined on the substrate, the solder would wet the substrate uncontrollably resulting in inconsistent formation or failure of the solder bumps and implicitly of the integrated circuit die.
To overcome this, usually a solder mask is formed on the facing surface of the substrate that has a plurality of openings corresponding to the solder bumps on the flip chip to limit the bump-bonding region on the substrate. The solder mask is used to protect (mask) certain areas of an integrated circuit board and confine the flow of solder during the soldering of connections to the circuit board and therefore prevent short-circuiting between solder bumps.
Increasing density and surface mount technology have increased the need for the solder mask to the point that, with the exception of the wettable pads, nearly all parts require the solder mask on the substrate.
One of the concerns with using the solder mask is the area on the substrate where the flip chip is mounted to the substrate. To design the openings in the solder mask that only leave the wettable pads exposed becomes difficult because of the increasingly tight tolerances.
Thus, a need still remains for a method to attach the flip chip to the substrate or printed circuit board to avoid solder contamination and bump collapse.
Additionally, the need to reduce costs, improve efficiencies and performance, and meet competitive pressures, adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.